As provided by the United States Constitution, each of the 435 members of the House of Representatives represents a district and serves a two-year term. House seats are apportioned among the states by population. The 100 Senators serve staggered six-year terms. Each state has two senators, regardless of population. Every two years, approximately one-third of the Senate is elected.

Article I of the Constitution vests all legislative power in the Congress. The House and Senate are equal partners in the legislative process (legislation cannot be enacted without the consent of both chambers); however, the Constitution grants each chamber some unique powers. The Senate is empowered to approve treaties and presidential appointments. Revenue-raising bills must originate in the House of Representatives, which also has the sole power of impeachment, while the Senate has the sole power to try impeachment cases.

The term Congress actually refers to a particular meeting of the national legislature, reckoned according to the terms of representatives. Therefore, a "Congress" covers two years. The current 110th Congress first convened on January 4, 2007.

Under the Articles of Confederation, which came into effect in 1781, the Congress of the Confederation was a unicameral body with equal representation among the states in which each state had a veto over most decisions. With no executive or judicial branch, and minimal authority given to the Congress, this government was weak compared to the states. The Congress of the Confederation had authority over foreign affairs and military matters, but not to collect taxes, regulate interstate commerce, or enforce laws. States remained sovereign, and were thus free to ignore any legislation passed by Congress. This system of government led to economic troubles in the states and disputes among the states.

The ineffectiveness of the federal government under the Articles of Confederation led the Congress to summon the Convention of 1787. Originally intended to revise the Articles of Confederation, it instead wrote a completely new constitution. Virginia delegate James Madison called for a bicameral Congress in his Virginia Plan: the lower house elected directly by the people, and the upper house elected by the lower house. The smaller states, however, favored a unicameral Congress with equal representation for all states; William Paterson countered Madison's proposals with the New Jersey Plan. Eventually, a compromise was reached: the House of Representatives was to provide representation proportional by population, whereas the Senate would provide equal representation by states. In order to preserve further the authority of the states, it was provided that state legislatures, rather than the people, would elect senators.

The Constitution gave more powers to the federal government, such as regulating interstate commerce, managing foreign affairs and the military, and establishing a national currency. These were seen as essential for the success of the new nation and resolve the disputes that had arisen under the Articles of Confederation, but the states retained sovereignty over other affairs. To protect against abuse of power at the federal level, the Constitution mandated separation of powers, with responsibilities divided among the executive, legislative, and judicial branches. Furthermore, the legislative body would be bicameral, so there would be checks and balances. The Constitution was ratified by the end of 1788, and its full implementation was set for March 4, 1789.

The post-Civil WarGilded Age was marked by Republican dominance of the Congress. The Progressive Era saw the Seventeenth Amendment (ratified in 1913), which provided for the direct election of senators. The early 20th century witnessed the rise of strong party leadership in both houses of Congress. In the House of Representatives, the office of Speaker became extremely powerful. Leaders in the Senate were somewhat less powerful; individual senators still retained much of their influence. After the revolt against Speaker Joe Cannon in 1910, the seniority system emerged. Members became powerful chairmen through years of seniority, regardless of the leadership. Committee chairmen remained particularly strong in both houses until the reforms of the 1970s and 1990s.

Franklin Delano Roosevelt's election as President in 1932 marked a shift in power towards the presidency. Numerous New Deal initiatives were proposed from the White House and sent to Congress for approval, rather than legislation originating in Congress. After the Watergate scandal and other abuses of power by the Nixon administration, Congress began to reassert its power to oversee the executive branch and develop legislation.

During the administration of President Franklin D. Roosevelt (1933–45), the Democratic Party controlled both houses of Congress. The Republicans won control of both houses in the 1946 elections, only to lose them in 1948; with Dwight D. Eisenhower's election to the presidency in 1952, the Republicans again won both houses. However, after the Democratic Party again won back control in the elections of 1954, it was the majority party in both houses of Congress for most of the next 40 years; the Republicans were only able to win control of the Senate for a six-year period (1981–87). The Republicans won a majority position in both houses of Congress in the elections of 1994. The Republicans controlled both houses until 2006, except the Senate for most of 2001 and 2002, when the Democrats had the majority after Jim Jeffords left the Republican Party to become an independent and caucus with the Democrats. In 2006, the Democratic Party regained control of the House of Representatives, and the results of the Senate elections yielded a Senate makeup of 49 Republicans, 49 Democrats, and two independents. In the 110th Congress (2007–08), the Democratic voting bloc has a 51-49 majority in the Senate because the two independents, Joseph Lieberman of Connecticut and Bernie Sanders of Vermont, align themselves with the Democratic Party.

Congress has authority over financial and budgetary matters, through the enumerated power to "lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defense and general welfare of the United States." (power of the purse) The Sixteenth Amendment extended power of taxation to include income taxes. The Constitution also gives Congress power over appropriating funds, with all government spending required to be included in congressional appropriations. This power is an important way for Congress to keep the executive branch in check. Other powers granted to Congress include the authority to borrow money on the credit of the United States, regulate commerce with foreign nations and among the states, and coin money.

The Constitution also gives Congress an important role in national defense, including the exclusive power to declare war, to raise and maintain the armed forces, and to make rules for the military. Congress also has the power to establish post offices and post roads, issue patents and copyrights, fix standards of weights and measures, establish courts inferior to the Supreme Court, and "To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof." Congress also has the power to admit new states to the Union (Article Four).

Implied powers

Congress also has implied powers derived from the necessary-and-proper clause of the Constitution which permits Congress "To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof." The Supreme Court has interpreted the necessary-and-proper clause broadly, to recognize the Congress has all the power and delegates it rather than being burdened with a separation of powers.

Checks and balances

The Constitution provides checks and balances among the three branches of the federal government. The authors of the Constitution expected the greater power to lie with Congress and it has been theorized that that is one reason they are described in Article One.

The influence of Congress on the presidency has varied from one period to another; the degree of power depending largely on the leadership of the Congress, political influence by the president, or other members of congress and the boldness of the president's initiatives. Under the first half-dozen presidents, power seems to have been evenly divided between the president and Congress, in part because early presidents largely restricted their vetoes to bills that were unconstitutional.

The Constitution concentrates removal powers in the Congress by empowering and obligating the House of Representatives to impeach federal officials (both executive and judicial) for "Treason, Bribery, or other high Crimes and Misdemeanors." The Senate is constitutionally empowered and obligated to try all impeachments. A simple majority in the House is required to impeach an official; however, a two-thirds majority in the Senate is required for conviction. A convicted official is automatically removed from office; in addition, the Senate may stipulate that the defendant be banned from holding office in the future.

Impeachment proceedings may not inflict more than this; however, the party may face criminal penalties in a normal court of law. In the history of the United States, the House of Representatives has impeached sixteen officials, of whom seven were convicted. (Another resigned before the Senate could complete the trial). Only two Presidents of the United States have ever been impeached: Andrew Johnson in 1868 and Bill Clinton in 1999. Both trials ended in acquittal; in Johnson's case, the Senate fell one vote short of the two-thirds majority required for conviction. In 1974, Richard Nixon resigned from office after impeachment proceedings in the House Judiciary Committee indicated he would eventually be removed from office.

The Constitution entrusts certain powers to the Senate alone. The President may only nominate for appointment Cabinetofficials, judges, and other high officers with the "by and with the advice and consent" of the Senate. The Senate confirms most presidential nominees, but rejections are not uncommon. Furthermore, treaties negotiated by the President must be ratified by a two-thirds majority vote in the Senate to take effect. The House of Representatives has no formal role in either the ratification of treaties or the appointment of federal officials, other than filling vacancies in the office of Vice-President.

In 1803, the Supreme Court established judicial review of federal legislation in Marbury v. Madison, holding, however, that Congress could not grant unconstitutional power to the Court itself. The Constitution does not explicitly state that the courts may exercise judicial review; however, the notion that courts could declare laws unconstitutional was envisioned by the founding fathers. Alexander Hamilton, for example, mentioned and expounded upon the doctrine in Federalist No. 78. Originalists on the Supreme Court have argued that if the constitution doesn't say something explicitly it is unconstitutional to infer what it should, might or could have said.

Investigations are conducted to gather information on the need for future legislation, to test the effectiveness of laws already passed, and to inquire into the qualifications and performance of members and officials of the other branches. Committees may hold hearings, and, if necessary, compel individuals to testify by issuing subpoenas. Witnesses who refuse to testify may be cited for contempt of Congress, and those who testify falsely may be charged with perjury. Most committee hearings are open to the public (the House and Senate intelligence committees are the exception); important hearings are widely reported in the mass media.

Legislative procedure

Term

The House of Representatives elects a Speaker to preside over debates. The President pro tempore of the Senate, by contrast, holds office continuously; normally, a new President pro tempore is only elected if the previous one retires, or if there is a change in the majority party.

A term of Congress is divided into two "sessions," one for each year; Congress has occasionally also been called into an extra, (or special) session. (The Constitution requires Congress to meet at least once each year.) A new session commences on January 3 (or another date, if Congress so chooses) each year. Before the Twentieth Amendment, Congress met from the first Monday in December to April or May in the first session of their term (the "long session"); and from December to March 4 in the second "short session". (The new Congress would then meet for some days, for the inauguration, swearing in new members, and organization.)

The Constitution forbids either house from meeting any place outside the Capitol, or from adjourning for more than three days, without the consent of the other house. The provision was intended to prevent one house from thwarting legislative business simply by refusing to meet. To avoid obtaining consent during long recesses, the House or Senate may sometimes hold pro forma meetings, sometimes only minutes long, every three days. The consent of both bodies is required for Congress's final adjournment, or adjournment sine die, at the end of each congressional session. If the two houses cannot agree on a date, the Constitution permits the President to settle the dispute.

Joint sessions

Joint Sessions of the United States Congress occur on special occasions that require a concurrent resolution from both House and Senate. These sessions include the counting of electoral votes following a Presidential election and the President's State of the Union address. Other meetings of both House and Senate are called Joint Meetings of Congress, held after unanimous consent agreements to recess and meet. Meetings of Congress for Presidential Inaugurations may also be Joint Sessions, if both House and Senate are in session at the time, otherwise they are formal joint gatherings.

At some time during the first two months of each session, the President customarily delivers the State of the Union Address, a speech in which he assesses the situation of the country and outlines his legislative proposals for the congressional session. The speech is modeled on the Speech from the Throne given by the British monarch, and is mandated by the Constitution of the United States—though it is not necessarily required to be delivered each year or in the customary manner. Thomas Jefferson discontinued the original practice of delivering the speech in person before both houses of Congress, deeming it too monarchical. Instead, Jefferson and his successors sent a written message to Congress each year. In 1913, President Woodrow Wilson reestablished the practice of personally attending to deliver the speech; few Presidents have deviated from this custom since.

Joint Sessions and Joint Meetings are traditionally presided over by the Speaker of the House except for the joint session to count electoral votes for President, when the Constitution requires the President of the Senate (the Vice President of the United States) to preside.

Bills and resolutions

A proposal may be introduced in Congress as a bill, a joint resolution, a concurrent resolution, or a simple resolution. Most legislative proposals are introduced as bills, but some are introduced as joint resolutions. There is little practical difference between the two, except that joint resolutions may include preambles but bills may not. Joint resolutions are the normal method used to propose a constitutional amendment or to declare war. On the other hand, concurrent resolutions (passed by both houses) and simple resolutions (passed by only one house) do not have the force of law. Instead, they serve to express the opinion of Congress, or to regulate procedure.

Members of Congress often introduce legislation at the behest of lobbyists. Lobbyists advocate the passage (or rejection) of bills affecting the interest of a particular group (such as a corporation or a labor union). In many cases, the lobbyists write legislation and submit it to a member for introduction. Congressional lobbyists are legally required to be registered in a central database, and are employed by political organizations, corporations, state governments, foreign governments, and numerous other groups. In 2005, there are almost 35,000 registered Congressional lobbyists, representing a doubling since 2000. Some of the most prominent lobbyists are ex-members of Congress, others are family members of sitting members. As an example, Harry Reid, Dennis Hastert, former Representative Tom DeLay, and Roy Blunt all have immediate family members who are (or were) lobbyists.

Bills (and other proposals) may be introduced by any member of either house. However, the Constitution provides that: "All bills for raising Revenue shall originate in the House of Representatives." As a result, the Senate does not have the power to initiate bills imposing taxes. Furthermore, the House of Representatives holds that the Senate does not have the power to originate appropriation bills, or bills authorizing the expenditure of federal funds. Historically, the Senate has disputed the interpretation advocated by the House. However, whenever the Senate originates an appropriations bill, the House simply refuses to consider it, thereby settling the dispute in practice. Nevertheless, while the Senate cannot originate revenue and appropriation bills, it does retain the power to amend or reject them.

Each bill goes through several stages in each house. The first stage involves consideration by a committee. Most legislation is considered by standing committees, each of which has jurisdiction over a particular subject matter, such as Agriculture or Appropriations. The House has twenty standing committees; the Senate has sixteen. In some cases, bills may be sent to select committees, which tend to have more narrow jurisdictions than standing committees. Each standing and select committee is led by a chair (who belongs to the majority party) and a ranking member (who belongs to the minority party). Committees are permitted to hold hearings and collect evidence when considering bills. They may also amend the bill, but the full house holds the power to accept or reject committee amendments. After considering and debating a measure, the committee votes on whether it wishes to report the measure to the full house.

A decision not to report a bill amounts to a rejection of the proposal. Both houses provide for procedures under which the committee can be bypassed or overruled, but they are rarely used. If reported by the committee, the bill reaches the floor of the full house. The house may debate and amend the bill; the precise procedures used by the House of Representatives and the Senate differ. A final vote on the bill follows.

Once a bill is approved by one house, it is sent to the other, which may pass, reject, or amend it. In order for the bill to become law, both houses must agree to identical versions of the bill. If the second house amends the bill, then the differences between the two versions must be reconciled in a conference committee, an ad hoc committee that includes both senators and representatives. In many cases, conference committees have introduced substantial changes to bills and added unrequested spending, significantly departing from both the House and Senate versions. President Ronald Reagan once quipped, "If an orange and an apple went into conference consultations, it might come out a pear. If both houses agree to the version reported by the conference committee, the bill passes; otherwise, it fails.

After passage by both houses, a bill is submitted to the President. The President may choose to sign the bill, thereby making it law. The President may also choose to veto the bill, returning it to Congress with his objections. In such a case, the bill only becomes law if each house of Congress votes to override the veto with a two-thirds majority. Finally, the President may choose to take no action, neither signing nor vetoing the bill. In such a case, the Constitution states that the bill automatically becomes law after ten days (excluding Sundays). However, if Congress adjourns (ends a legislative session) during the ten day period, then the bill does not become law. Thus, the President may veto legislation passed at the end of a congressional session simply by ignoring it; the maneuver is known as a pocket veto, and cannot be overridden by the adjourned Congress.

Every Act of Congress or joint resolution begins with an enacting formula or resolving formula stipulated by law. These are:

Act of Congress: "Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled."

Joint resolution: "Resolved by the Senate and House of Representatives of the United States of America in Congress assembled."

Quorum and vote

The Constitution specifies that a majority of members constitutes a quorum to do business in each house. The rules of each house provide that a quorum is assumed to be present unless a quorum call demonstrates the contrary. Representatives and senators rarely force the presence of a quorum by demanding quorum calls; thus, in most cases, debates continue even if a majority is not present.

Both houses use voice voting to decide most matters; members shout out "aye" or "no," and the presiding officer announces the result. The Constitution, however, requires a recorded vote on the demand of one-fifth of the members present. If the result of the voice vote is unclear, or if the matter is controversial, a recorded vote usually ensues. The Senate uses roll call votes; a clerk calls out the names of all the senators, each senator stating "aye" or "no" when his or her name is announced. The House reserves roll call votes for the most formal matters, as a roll-call of all 435 representatives takes quite some time; normally, members vote by electronic device. In the case of a tie, the motion in question fails. In the Senate, the Vice President may (if present) cast the tiebreaking vote.

Committees

It is neither expected nor possible that a member of Congress be an expert on all matters and subject areas that come before Congress. Congressional committees provide invaluable informational services to Congress by investigating and reporting back in regard to specialized subject matter.

While this investigatory function is indispensable to Congress, procedures such as the House discharge petition process (the process of bringing a bill onto the floor without a committee report or mandatory consent from its leadership) are so difficult to implement that committee jurisdiction over particular subject matter of bills has expanded into semi-autonomous power. Of the 73 discharge petitions submitted to the full House from 1995 through 2007, only one was successful in securing an definitive yea-or-nay vote for a bill on the floor of the House of Representatives. Not without reason have congressional committees been called independent fiefdoms.

In 1931 a reform movement did temporarily reduce the number of signatures required on discharge petitions in the U.S. House of Representatives from a constitutional majority of 218 down to 145, i.e. from one-half to one-third of the House membership. This reform was abolished in a 1935 counterattack led by the intra-House oligarchy. Thus the era of the Great Depression marks the last across-the-board change, albeit a short-lived one, in the autonomy of House standing committees. On strategy for an enduring reform in the system of semi-autonomous committees see the citation.

In the course of committee work, members will often develop personal expertise on the matters under the jurisdiction of their respective committee(s). Such expertise, or claims thereof, are invariably cited during disputes over whether the parent body should bow to obdurate committee negatives.

Decision on which areas individual members choose to specialize may be influenced by their constituency and regional issues of importance to them, as well as prior background and experience of the member. Senators will also try to differentiate themselves from the other senator from the same state, so that areas of specialization do not overlap.

Constituent services

A major aspect of the job for a Senator and a Congressman consists of services to his or her constituency. Members receive thousands of letters, phone calls, and e-mails, with some expressing opinion on an issue, or displeasure with a member's position or vote. Other constituents request help with problems, or ask questions. Members of Congress want to leave a positive impression on the constituent, rather than leave them disgruntled. Thus, their offices will be responsive, and go out of their way to help steer the citizen through the intricacies of the bureaucracy. Here the Congressman and his staffers perform the function of an Ombudsman, at the Federal level. This unofficial job has become increasingly time consuming, and has significantly reduced the time that Congressmen have for the preparation or inspection of bills.

It is noteworthy that an incumbent member of Congress has considerably more clout than most official ombudsmen at the state level, and in other countries, given the appointive and relatively diminutive character of such offices. As Morris Fiorina notes, the involvement of the legislative branch in the ombudsman process carries one major advantage: members of Congress exercise "control over what bureaucrats value most – higher budgets and new program authorizations. This kind of leverage over the bureaucracy is a potent tool that appointed ombudsmen lack.

Accordingly, to improve on today's 435 de facto ombudsmen -- constituent services by overworked Congressmen -- congressional reforms have been proposed that would approximate the legislative leverage now exercised by Congressmen, but in an office where the intra-bureaucratic troubleshooting duties are full time. Along these lines, some Congressmen themselves have suggested that each congressional district should elect a second U.S. Representative to handle constituent services.

Privileges

Under the Constitution, members of both houses enjoy the privilege of being free from arrest in all cases, except for treason, felony, and breach of the peace. This immunity applies to members during sessions and when traveling to and from sessions. The term "arrest" has been interpreted broadly, and includes any detention or delay in the course of law enforcement, including court summons and subpoenas. The rules of the House strictly guard this privilege; a member may not waive the privilege on his or her own, but must seek the permission of the whole house to do so. Senate rules, on the other hand, are less strict, and permit individual senators to waive the privilege as they see fit.

The Constitution also guarantees absolute freedom of debate in both houses, providing, "for any Speech or Debate in either House, they shall not be questioned in any other Place." Hence, a member of Congress may not be sued for slander because of remarks made in either house. However, each house has its own rules restricting offensive speeches, and may punish members who transgress them.

Obstructing the work of Congress is a crime under federal law, and is known as contempt of Congress. Each house of Congress has the power to cite individuals for contempt, but may not impose any punishment. Instead, after a house issues a contempt citation, the judicial system pursues the matter like a normal criminal case. If convicted in court, an individual found guilty of contempt of Congress may be imprisoned for up to one year.

From 1789 to 1815, members of Congress received only a per diem (daily payment) of $6 while in session. Members began receiving an annual salary in 1815, when they were paid $1,500 per year.

As of 2006, rank-and-file members of Congress received a yearly salary of $165,200. Congressional leaders are paid $183,500 per year. The Speaker of the House of Representatives earns $212,100 per annum. The salary of the President pro tempore for 2006 is $183,500, equal to that of the majority and minority leader of the House and Senate.

Members elected since 1984 are covered by the Federal Employees Retirement System (FERS). Those elected prior to 1984 were covered by the Civil Service Retirement System (CSRS). In 1984 all members were given the option of remaining with CSRS or switching for FERS. As it is for all other federal employees, congressional retirement is funded through taxes and the participants' contributions. Members of Congress under FERS contribute 1.3% of their salary into the FERS retirement plan and pay 6.2% of their salary in Social Security taxes.

The amount of a Congressperson's pension depends on the years of service and the average of the highest 3 years of his or her salary. By law, the starting amount of a Member's retirement annuity may not exceed 80% of his or her final salary. In 2006, the average annual pension for retired senators and representatives under CSRS was $60,972, while those who retired under FERS, or in combination with CSRS, was $35,952.

Another privilege is the use of the Library of Congress. One of the Library's missions is to serve the Congress and its staff. To do this, the Congressional Research Service provides detailed, up-to-date and non-partisan research for senators, representatives, and their staff to help them carry out their official duties. The franking privilege allows members of Congress to send official mail to constituents at government expense. Though, they are not permitted to send election materials, borderline material is often sent, especially in the run-up to an election by those in close races.

A legislator in either house is a "member of Congress," though usually only a representative are referred to in speech as a congressman, congresswoman, or congressperson.

Comparison with parliamentary systems

Many of the world's democracies and republics operate not within a congressional model of government, but rather a parliamentary system. The most significant difference between a parliamentary government and the U.S. Congress is that a parliament typically encompasses the entire governmental regime, containing legislative, executive, and judicial branches within its structure (the executive organs are often referred to as "The Government"), as well as the monarch, if one exists. The U.S. Congress exercises only legislative powers, and is but one of three co-equal and independent branches of the larger federal government.

In a parliament, the executive branch of the government is chosen from or by the representative branch. This generally comprises the prime minister and the governing cabinet. Congressional leaders merely administrate the daily business of Congress itself, while it is in session, and not the functioning of the national government as a whole. So, while in structure the Speaker of the House of Representatives resembles a prime minister, in substance and practice he or she only moderates the functioning of the U.S. Congress, while the wholly separate executive branch of government administrates the daily functioning of the federal government. In the U.S. Congress, legislation originates within the legislative branch, whereas in a parliamentary system, legislation is drafted by the government in power and then sent to parliament for debate and ratification.

Members of the U.S. Congress are generally elected from one of two parties, but its members are free to vote their own conscience or that of their constituents. Many members can and do cross party lines frequently. In a parliamentary system, members may be compelled to vote with their party's bloc, and those who vote against are often cast out of their respective parliamentary parties and become less influential independents. Theoretically, the lack of superpowerful political parties allows U.S. members to more faithfully represent their constituents than members of parliament can—a member is ultimately responsible to their constituents alone, not to their party. Additionally, as Congress does not wield executive power, dissenting votes from the majority party cannot result in the collapse of the ruling Government and new elections, as occasionally happens in parliamentary systems. Conversely, the congressional system also allows for a larger role for extra-governmental actors such as lobbyists, as the lack of strong party whips present in parliamentary systems exposes members of Congress to greater outside influence.

The offices of Speaker of the House of Representatives and Speaker of the House of Commons, while holding very similar titles and presiding over their respective bodies, differ greatly. In the British House of Commons, the speaker is officially and actively non-partisan, and must resign from his or her party upon accepting the office. His or her role duties in-camera include ensuring active and fair debate, and recognising members of all parties, including back-benchers. Ex-camera, the Speaker of the House of Commons supervises the support offices of Parliament. By contrast, the Speaker of the House of Representatives is the leader of his or her party's members in the House (senior even to the Majority Leader) and has considerable authority to appoint members to committees and otherwise reward or punish members. When the President of the United States is not of the same party as the majority in House of Representatives, the Speaker is the de facto "leader of the opposition."